Antimicrobial susceptibilities of a Corynebacterium CDC group I1 strain isolated from a patient with endocarditis

We encountered a case of native valve endocarditis due to Corynebacterium CDC group I1 which was successfully controlled with antimicrobial agents alone. This organism and three other isolates of this group were susceptible to penicillin, vancomycin, and gentamicin. The combination of penicillin with subinhibitory concentrations of gentamicin resulted in a 1,000-fold decrease in CFU per milliliter at 24 h compared with penicillin alone. Augmentation of killing was noted also with vancomycin plus gentamicin but to a degree that did not meet strict criteria for bactericidal synergism.